Advances in plasma processing have provided for growth in the semiconductor industry. To supply chips for a typical electronic product, hundreds or thousands of substrates (such as semiconductor wafers) may be processed. In order for the manufacturing company to be competitive, the manufacturing company needs to be able to process the substrates into quality semiconductor devices with minimal processing time.
Typically, during plasma processing, problems may arise that may cause the substrates to be negatively impacted. One important factor that may alter the quality of the substrate being processed is the plasma itself. In order to have sufficient data to analyze the plasma, sensors may be employed to collect processing data about each substrate. The data collected may be analyzed in order to determine the cause of the problems.
To facilitate discussion, FIG. 1 shows a simple schematic diagram of a data collecting probe in a portion of a plasma system 100. Plasma system 100 may include a radio frequency (RF) source 102, such as a pulsating RF frequency generator, capacitively-coupled to a reactor chamber 104 to produce plasma 106. When RF source 102 is turn on, a bias voltage is developed across an external capacitor 108, which may be about 26.2 nanofarads (nF). In an example, RF source 102 may provide a small burst of power (e.g., 11.5 megahertz.) every few milliseconds (e.g., about five milliseconds) causing external capacitor 108 to be charged. When RF source 102 is turned off, a bias voltage remains on external capacitor 108 with a polarity such that probe 110 is biased to collect ions. As the bias voltage decays, the curves as shown in FIGS. 2A, 2B and 3 may be traced.
Those skilled in the art are aware that probe 110 is usually an electrical probe with a conducting planar surface that may be positioned against the wall of reactor chamber 104. Probe 110 is thus directly exposed to reactor chamber 104 environment. Current and voltage data collected by probe 110 may be analyzed. Since certain recipe may cause a non-conducting deposition layer 116 to be deposited on probe 110, not all probes may be able to collect reliable measurements. However, those skilled in the art are aware that a PIF (planar ion flux) probe enables data to be collected despite the non-conducting deposition layer since the PIF probe scheme is not required to draw a direct current (DC) to implement a measurement.
The current and voltage signal in plasma system 100 is measured by other sensors. In example 100, when RF source 102 is switched off, current sensor 112 and a high impedance voltage sensor 114, are employed to measure die current and the voltage, respectively. The measurement data collected from current sensor 112 and voltage sensor 114 may then be plotted to create a current graph and a voltage graph. The data may be manually plotted or the data may be entered into a software program to create the graphs.
FIG. 2A shows a graph of voltage versus time after a RF charge cycle. At data point 202, RF source 102 has been switched off after an RF charge has been provided (i.e., RF burst). In this example, at data point 202, the voltage across probe 110 is about negative 57 volts. As plasma system 100 returns to a rest state (interval between data points 204 and 206), the voltage usually reaches a floating voltage potential. In this example, the floating voltage potential rises from about negative 57 volts to about zero volt. However, the floating voltage potential does not have to be zero and may be a negative or a positive bias voltage potential.
Similarly, FIG. 2B shows a graph of current data collected after a RF charge. At data point 252, RF source 102 has been switched off after an RF charge has been provided. During a decay period 254, the return current at external capacitor 108 may be discharged. In an example, at full charge (data point 252), the current is about 0.86 mA/cm2. However, when the current is fully discharged (data point 256), the current has returned to zero. Based on the graph, the discharge takes about 75 milliseconds. From data point 256 to data point 258, the capacitor remains discharged.
Since both the current data and the voltage data are collected over a period of time, a current versus voltage graph may be generated by coordinating the time in order to eliminate the time variable. In other words, the current data collected may be matched against the voltage data collected. FIG. 3 shows a simple current versus voltage graph for a single time interval between a RF burst. At data point 302, RF source 102 has been switched off after an RF charge has been provided.
By applying a non-linear fit to the data collected during each RF burst, plasma 106 may be characterized. In other words, parameters (e.g., ion saturation, ion saturation slope, electron temperature, floating voltage potential, and the like) that may characterize plasma 106 may be determined. Although plasma 106 may be characterized with the data collected, the process of calculating the parameters is a tedious manual process that requires human intervention. In an example, when the data has been collected after each RF burst (i.e., when the RF charge has been provided and then turned off), the data may be fed into a software analysis program. The software analysis program may perform a non-linear fit to determine the parameters that may characterize the plasma. By characterizing the plasma, the engineer may be able to determine how a recipe may be adjusted in order to minimize substandard processing of the substrates.
Unfortunately, the prior art method of analyzing the data for each RF burst may require several seconds or as much as several minutes to complete. Since there are typically thousands, if not millions of RF bursts to analyze, the total time for characterizing the plasma for a recipe may take hours to calculate. As a result, the prior art method is not an effective method in providing timely relevant data for process control purposes.